Abstract: A method of detecting a mutation of a specific nucleotide base in a target nucleic acid chain comprises: (a) hybridizing a labelled probe to the target to form a hybrid in which one end of the probe is positioned adjacent the specific base; (b) adding a nucleotide derivative, e.g. a thionucleotide, under conditions to cause it to join to the end of the probe if it is complementary to the specific base; (c) digesting the hybrid using an exonuclease enzyme under conditions such that the nucleotide derivative protects the probe from digestion; and observing the presence or absence of label attached to the target. The method can be used to detect mutations even when these are not present at restriction enzyme cleavage sites, and does not require the preliminary steps of restriction digestion, gel electrophoresis and DNA (or RNA) blotting.